The unit processes that are commonly utilized in the treatment of waste water have changed only moderately over the last fifty years. The changes have been primarily in design of apparatus, operating techniques and materials, resulting in increased dependability and efficiency. The intent and function of the unit processes remained unchanged.
The development of the "pulsed bed filter" and the "hydraulically regenerated filter" both described in U.S. Pat. Nos. 3,459,302; 3,516,930; 3,587,861; 3,792,773; 3,817,378; 3,840,117; 4,032,443; 4,627,923; 4,693,831; Re. 28,458 and Canadian Pat. No. 1,181,698 have made the unit process of primary filtration a practical, viable, and an effective unit process.
The development and application of primary filtration as a unit process has prompted the in-depth study of the size, character, distribution and polluting effect of the solids suspended in waste water prior to and following filtration. These studies involved the character and quantities of suspended particles that can be removed by the filtration process and their effect in the water treatment process, as well as the character, quantities, treatment and polluting effect of the solids retained within the filtrate.
The analytical standard method for the examination of water and waste water is prepared and published jointly by the American Public Health Association, the American Water Works Association, and the Water Pollution Control Federation. The sixteenth edition, published in 1985, describes the standard and accepted method of suspended solids determination. The key element in the determination is the use of a glass fiber filter disk such as Whatman grade 934AH, Gelman A/E, or a Millipore AP40. These filter disks are generally effective in the removal of solids greater than 1.2 microns. The suspended solids that pass through the filter disk have been considered nonexistent and labeled as soluble, and a biochemical analysis of the filtrate is referred to as the soluble BOD.sub.5 or the soluble carbonaceous biological oxygen demand (CBOD).
The utilization of the pulsed bed or surface regeneration filter in the filtration of primary effluent results in substantial reductions of suspended solids, and a corresponding decrease in the organic residual in the filtrate. The filter performance can be expressed by BOD.sub.5 and suspended solids evaluation of influent and filtrate.
In a study conducted at University of California, Davis, presented at the 53rd Annual Water Pollution Control Federation 1980 meeting, Matsumato, et al established that suspended solids removals by the primary filtration process (as determined by Standard Methods) are predictable. He reported removals of 30 to 75% depending on sand size and application rate. BOD.sub.5 removal also varied from 28% to 63% under the same conditions.
The study printed in the Journal Water Pollution Control Federation, p. 1581-1591, Dec. 1982, contained an analysis (in FIG. 11) of solids contained in primary effluent, and in primary filtrate utilizing NUCLEOPORE polycarbonate filter papers. A significant conclusion is the fact that a relatively high quantity of solids, 1 micron and smaller, that are contained in the primary influent remain in the primary filtrate. This same study also establishes an effective removal of most of the solids greater than 2 microns in size utilizing 0.45 mm effective size sand. A detailed study entitled "Characterization of the Size Distribution of Contaminants in Wateswater: Treatment and Reuse Implications", by Levine, et al was presented at the 57th Annual Conference Water Pollution Control Federation in 1984 and published in the Journal Water Pollution Control Federation, p. 805-816, July, 1985. This study presented information on the impact of treatment processes on the size distribution of particles. This study also agreed with other researchers that particles smaller than 1.2 micron are generally not measured by the standard suspended solids tests, and a large number of bacterial cells, cell fragments, viruses and inorganic particles such as clay are in the size of 0.1 to 1.2 microns.
An earlier study by Balmat, published in Sewage and Industrial Wastes, Vol. 29, No. 7, p. 757-761, July, 1957, classified organic materials as soluble (&lt;0.8 micron) colloidal (0.08 to 1.0 micron) supra-colloidal (1-100 micron) and settleable &gt;100 micron. Balmat studied the oxidation rate of the various organic materials as referenced to size and determined that the biochemical oxidation rate k d.sup.-1 (base, 10) increased from 0.09 and 0.08 for particles size over 1 micron to 0.22 for particles 0.08 to 1.0 micron.
It has been established in the reported studies that primary filtration is very effective in the removal of solids greater than 2 microns, with only minor removals of smaller particles. This change in the distribution of filtrate particles by the primary filtration process also results in a very rapid oxidation of the residual organics in the filtrate and allows rapid new cellular growth. This rapid growth rate is most desirable in downstream biological processes, but can be process defeating if uncontrolled. The new cellular materials have a tendency to attach to surfaces and may be recognized by their slimy appearance and reported as slime.
Primary filters have conventionally utilized their own filtrate to backwash, regenerate and clean the primary filter media. However rapid growth of cellular materials can, if not controlled, adversely effect the dependability and integrity of the primary filter system.
The term regenerate refers to the act of turning over of the uppermost layer of media in the filter bed to present clean media to the wastewater and to accumulate filtered separated solids within the filter bed. The "pulsed bed filter" regenerates the media bed by forcing filtrate into the vented underdrain chamber, and compressing the air within the filter support structure, which forces air upwardly through the bed to cause regeneration. The "hydraulically regenerated filter" employs an array of conduits within the filter bed having upwardly directed jets which regenerate the media surface by pumping filtrate through the conduits and jet assembly. Regeneration in both filters leaves the media bed essentially in tact and prolongs the filter run.
After a number of regeneration cycles, filter capacity has decreased to the point that the solids in the bed must be removed by backwashing the bed with filtrate. Both filters force filtrate into the underdrain chamber, up through the support screen and filter media, expanding and fluidizing the media and washing away the stored solids with the backwash filtrate. The combination of regeneration and backwashing may be considered together as renewal of the media bed filter.
A recent study entitled "Evaluation of a Pulsed Bed Filter for Filtration of Municipal Primary Effluents" by Donald S. Brown, U.S.E.P.A. and published in the Journal Water Pollution Control Federation. p. 72-78, Feb. 1987 pointed out minor problems that had the potential to become major problems. One of the problems with the greatest potential to become a major problem was the rapid growth of biological slime. Further, as the growth builds on the media support membrane, uneven upward flow of backwash liquid produces uneven cleaning and reduced filter efficiency. Eventually buildup of growth on the membrane will plug the membrane, impede backwash liquid flow and perhaps even rupture the support membrane resulting in total failure of the filter. The use of primary filtration has been in operation for many years at Amherst, Ohio where the problems of biological growth were confirmed and studied. The problems pointed out by Brown also existed and were duplicated at Amherst, Ohio, and have been eliminated by my improvement in the apparatus and methodology of primary filtration.
This invention is not limited to use with "pulsed bed filters" and "hydraulically regenerated filters". In recent years filters with designs other than those described in my patents have been used to treat primary waste or highly organic waste. The surfaces of all granular material are ideal sites for attached growth activity. The backwashing of any such media filter with retained filtrate contaminated with new cellular growth can cause further difficulties by the solids-laden backwash being introduced upward through the support media allowing the bacterial growth to attach to the granular support layers, particularly the finer media particles, resulting in decreased filter performance and possible filter failure. Further, other media support systems may contain surfaces other than, or in addition to, granular material. These alternatives include blocks, flumes, tubes, and other curved and flat surfaces that also present ideal sites for attached growth. Periodic flushing of the underdrain systems with solids-free bactericide-treated liquid, as described in my invention, will retard the growth on these underdrain surfaces as well as on the granular systems, and the fine media system support.